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RV Hauler Axle and hitch placement


Jack Mayer

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Gregg and I shot a video explaining the placement of the hitch on a typical RV hauler based on an HDT. Many people ask about placing the hitch behind the axle, and also about "un-weighing" the front axle. Take a look at the video, and I'd love to hear your comments. This is a good discussion topic for this group.

 

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One correction. Singling long will increase turning radius not decrease. Easily fixed in video with a pop up bubble. Now I have some more math to do. What would you suggest as the lightest you would want to go on the front axle lb wise.

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Jack,

 

I don't get it . . . you mean the front wheels don't bounce up off the ground when hitched up? Just kidding. Good job explaining and great idea to both you guys for making the video.

 

Our truck is similar to your examples and we unload the front suspension about 1,000 lbs when hitched. This leaves about 11,000 lbs sitting on our 13,200 lb rated front axle.

 

As you noted, there are some dimensions to consider when designing your bed. For us, we wanted a 24” deep Drom storage box. From the face of the Drom to king pin we spec'd 130” to allow for the wider Smarts and plenty of fifth wheel turning space. These two measurements drove the overall length of Ruby. We singled at 235” which gave us reasonable load distribution. Couldn’t be happier with how it tows and rides.

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Jack

I saw the video yesterday. To dispel the myth that putting the axle behind the hitch is somehow unsafe, I think the most common commercial application of the 5th wheel behind the axle is car hauling where the hitch is mounted behind the truck tires down low. Those axle weights are all over the place as you pick up and deliver cars and I don't hear those guys complaining about braking and handling.

 

Nigel

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Jack, great explanation on the hitch placement. I ordered my truck with a 13.2K front axle mainly because aside from the smart car on the deck the 2 x 150 gallon tanks were moved forward much further than most commercial applications. This loads the front axle even more. The full fuel tanks weigh more than the smart car. The moved back hitch pin weight of the RV is welcome and is much nicer to drive than my 1 ton dually with the pin weight over the rear axle.

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Question Jack,

So does this mean that singling short might cause safety issues, based on the mechanical leverage?

The Heavy or Heavier rigs (RV's) that are married up to a hitch with an extended frame behind a singled short truck , do they poses real issues? ***PLEASE correct me if I am wrong!!

 

Another Question..

If a (for example) Volvo truck is singled short "as illustrated in the video", this would not an be ideal or safe set up, because the rear axle might become overloaded due to the mechanical leverage, and causing the front steers to become too light..

 

So the only RV Haulers that obviously can be sold/bought safely and legally, would be trucks that are singled mid and single long.. Am I correct, or am I close?

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The statement that most semi's run less weight thatn"we" do is misleading. They run less percentage of their weight on the front axle but because their gross is so much higher, they run more weight on the front axle than the average RV'er.

Not really much more on a semi than the heavier RV haulers. A semi is going to push the front end whenever loaded (as a generalization). My front, loaded, is pretty heavy. An RV Hauler is still going to be running between 9400 lbs (absolute worst case measured) and 11K+ on the front (more typical). That is not going to affect anything noticeably. And that is proved out in usage scenarios. Yes, they run more weight, but not that much more. And the other forces acting on the geometry are way more extreme.

 

As to a short singled truck, it really is not an issue. Or at least it has not proven to be, as LONG AS you have enough pin weight and do not have a lot of cantilevered force acting at the back of the trailer. Where I would worry about a short singled truck - or really any RV hauler - is when there is a very light pin weight relative to the total trailer weight, AND there is a lot of weight in the back of the trailer. Specialized Toyhaulers carrying many thousands of pounds in the garage have proven to be an issue. But that is not related to a short singling, but to the massive movement of force from the rear of the trailer acting on the front pin area. Set that in motion and you can have some "issues". In practice Henry and I have seen a few pretty severe examples of poor handling in that situation.

 

On the other hand, with a conventional trailer at 21+K lbs and 5000+ lb pin weight on a 182" wheelbase singled short Volvo 610 there were zero issues. Even running a heavier trailer at 23K and 6K+ pin I had no problems. That is over 120+K miles and 10+ years of fulltiming with those rigs. That tractor handled excellently in heavy winds and on poor condition roads. So given that other parameters were in normal ranges I see no issue with short singled trucks.

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Ref the Single short question

 

We run a 2001 Volvo 770 singled Short to 204" WB. Carry Smart and hitch placed behind axle

 

Loaded wts on truck w/ smart and 4800lbs pin

front 9800lbs

Rear 17400

 

Unloaded wts on truck w/smart no trailer

Front 11300

Rear 11400

 

Running these wts since loading smart 2009.

 

No adverse issues at all.

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No adverse issues at all.

No, and you should not have any. Those who believe that these trucks are "death traps" with the hitch behind the axle center line are simply misinformed. They need education, which is part of the reason we made the video.

 

That said, IF you do a very light pin weight WITH a very heavy rear weight in the trailer, you are going to have issues. But that would be true no matter what.

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Not really much more on a semi than the heavier RV haulers. A semi is going to push the front end whenever loaded (as a generalization). My front, loaded, is pretty heavy. An RV Hauler is still going to be running between 9400 lbs (absolute worst case measured) and 11K+ on the front (more typical). That is not going to affect anything noticeably. And that is proved out in usage scenarios. Yes, they run more weight, but not that much more. And the other forces acting on the geometry are way more extreme.

 

As to a short singled truck, it really is not an issue. Or at least it has not proven to be, as LONG AS you have enough pin weight and do not have a lot of cantilevered force acting at the back of the trailer. Where I would worry about a short singled truck - or really any RV hauler - is when there is a very light pin weight relative to the total trailer weight, AND there is a lot of weight in the back of the trailer. Specialized Toyhaulers carrying many thousands of pounds in the garage have proven to be an issue. But that is not related to a short singling, but to the massive movement of force from the rear of the trailer acting on the front pin area. Set that in motion and you can have some "issues". In practice Henry and I have seen a few pretty severe examples of poor handling in that situation.

 

On the other hand, with a conventional trailer at 21+K lbs and 5000+ lb pin weight on a 182" wheelbase singled short Volvo 610 there were zero issues. Even running a heavier trailer at 23K and 6K+ pin I had no problems. That is over 120+K miles and 10+ years of fulltiming with those rigs. That tractor handled excellently in heavy winds and on poor condition roads. So given that other parameters were in normal ranges I see no issue with short singled trucks.

 

 

As some folks may recall at times it seems that I have a HDT RV Weight and Balance fetish, and I sorta do, however for selfish reasons I NEED to keep a eye on our somewhat unconventional RV configuration so that the entire combo retains reasonable Weight and Balance status.

 

On New Years holiday I started a thread here regarding some calculation methods that may be utilized to test various RV combos (tractor / trailer/ hitch) Weight and Balance What-If loadings.....before the process of building hauler beds and locating hitches.

 

Unfortunately winter on the interior Western U S has become a somewhat grim winter and I had to leave our sunny Last Chance Peak Dolly horse camp and slog up into very grim snow and ice storms to deal with way too much real estate......ugh...

Anyhow I am still up in crappy-weather-Oregon but hope to escape sometime next week and then after catching up on my stable-boy duties at Last Chance Peak I hope to resume my What-If-Loadings-Calculations on the thread that I started at the first of the year. What I hope to do with the thread is to perhaps shed some light on various methods of configuration-loading that might result in a better Weight and Balance status for our RV units.

 

Jack and Henry and others have commented some regarding some of the larger newer 5ers that have low teen-% pin weights relative to total trailer weight and for the most part this often can result in less that ideal handling while towing. If life were only simple as attaining a golden % of pin weight we could all retire with no worries.

Having to sit on the back of the room and listen to a bunch of big-brain vehicle-dynamics-geeks calculate and test various effects of Weight and Balance ..AND....Load-Distribution I would never cease to be amazed how a group of nearly identical vehicle Weight and Balance conditions could handle vastly different by way the loads were distributed in the vehicle. Jack comment regarding high concentration of weight at the far aft end of a trailer is a example of a less-than-ideal-load-distribution of some RV trailers and sometimes this configuration can be a contributing factor in adverse tow handling. Better-practices hold that a less concentrated load mass aft is better achieved by breaking the load into smaller portions and evenly distributing the mass over a longer arm of the trailer.

 

The one subject that has not come up is dampening of less-stable vehicle-dynamics....as the RV trailer becomes mass-critical in the aft section the ability of the trailer suspension and tires to dampen unstable motion become very important. So while pin weight is important, it is just one of many things to consider in the overall handling of the RV combo.

 

Some folks seem prone to try to generalize on IF it is Safe or Legal to single a HDT.short. If only life was that simple.....too many variables are to be considered and factored to make any blanket statement such as that. For the most part I suspect that Weight and Balance adverse conditions in the majority of HDT haulers are fairly well tolerated in the tractor safety margins.....this is not to condone adverse Weight and Balance operations but is likely that the HDT tractor likely has better overload margins than any other RV hauler.

 

What I will state is that IF you calculate the Weight and Balance numbers on the vast numbers of pickup towed larger RV trailers it is a very sobering set of numbers that often appear on the spreadsheet.

 

If I ever escape Oregon.......,

 

Drive on.......(where is that darn snow blower....)

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Here's the longest conversion I was involved in building.

Dab%20Dick%203_zpsipy0u1xu.jpg

 

Dab%20Dick%205_zpssd8nq1ia.jpg

 

That's what the customer wanted, that's what the customer got. Two parameters at play here, a Smart car with mirrors out is about 6 feet in width, you have to clear it by about 5 feet, the hitch head better be about 11 feet back. Any subcompact, two seater, or a short Jeep are 12 feet long, you want to drive them from the rear, you better have 17 feet to the hitch head to clear things. Building it meant that for starters we had to add 6 feet of rails behind the rear axle.

 

Dab%20Dick%202_zps1x91phd5.jpg

 

There are few owners who have done something similar to carry car lengthwise. This owner of this one reported that he felt going around corners like the trailer was "wagging the truck". Do others who have long rigs like this experience "such feeling"? It would be interesting to get "another report"

This truck was eventually sold and it no longer has this bed, it was replaced with much shorter hauler body since the owner had other plans for the use of the truck.

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Wouldn't it be nice if there were plenty of campgrounds that would handle slightly longer rigs, and if we could legally have trailers of 48' or even 53'? We could stay tandem, use real truck suspensions/wheels on the trailer, placed farther back, and nearly all these weight/balance issues would go away. Stacker toy haulers would be practical (forget the smart, take the vette), living space would be plentiful, storage issues would, okay, we'd still have more stuff to fill that space..... Heck, we could load so heavy we wouldn't need an air hitch because the tractor would be working closer to design capacity, as it should.

 

Now back to the real world...........

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In case this helps anyone, here are my bed dimensions and truck configuration:

 

Wheelbase: 220"

Front axle weight rating: 12k lbs

Tandem rear axle weight rating 40k lbs (20k lbs each axle)

Bed length: 18'

Bed width: 8'

Bed material: 1/4" steel

Frame rail extension: 6'

5th wheel hitch: ET Senior

Bumper pull hitch: Standard 2" square supported by 1/2" plate with extra reinforcing gussets. Very over built.

 

The fifth wheel hitch head sits just below the level of the bed when aired up. The bed has a hinged section that covers the hitch when not in use.

 

The bumper pull hitch is placed as close to the support plate as possible and still have just enough room to get the hitch pin in and out. That's to make it both as short as possible as well as get the tongue weight as close to the support as possible.

 

I use an AirSafe Class V receiver hitch to provide some isolation between the truck and trailer.

 

On the bed I normally carry a full sized vehicle that is about 15' long and weighs roughly 8,500 lbs (I have not weighed the "toad" in its current configuration, so that is a rough guess). Its center of gravity is just forward of the center line of the rearmost axle.

 

The trailer that I pull is a tag-along (bumper pull) that weighs roughly 9k lbs with roughly a 1k lbs tongue weight.

 

The balance is not ideal as I would rather have the tandem axles a little further back, but the turning radius has been outstanding and the long swing on the hitch has enabled me to maneuver the trailer through spots that other RVs have difficulty navigating.

 

These are the scale #'s from a couple of months ago:

 

Truck Front Axle: 7,300

Truck Rear Axles: 29,860

Trailer Axles: 8,180

Total: 45,340

 

The truck is very stable. The only difference in driving it empty versus loaded is it takes a little longer to accelerate and slow down. Otherwise, it drives and handles the same. The front end doesn't "feel" light. No wandering, the trailer definitely doesn't push the truck around (it's a light trailer compared to what most of you are pulling), and the road ruts don't affect it any more than normal.

 

I did drive the truck a few thousand miles before the bed was put on it and without towing anything. It subjectively feels the same to me now as it did before I modified it.

 

The weight distribution is not ideal in my opinion. I would like to have a little more on the fronts. But it works for me.

 

I have never used the 5th wheel hitch, so I can provide no feedback on that aspect.

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I added 5' to the rails at the end of out HDT to carry a Subaru Forester. Our HDT is a 2001 Volvo with a 42" cab and an ISX. This truck handles great. Unfortunately I don't have weight figures. There are 2 100 gallon tanks, a Yamaha generator, space for a RZR and tool boxes all around. The bed is pretty heavy. We pull a 39' Teton. We have traveled with and without the car and RZR with no noticeable difference. I pulled a couple of semi trailers with this truck before I built the bed and the way it handles today is the same as before. I have been reading this thread and I wonder what the problem is with the hitch this far back. I have pulled some heavy gooseneck trailers and bumper pull trailers behind pickups that unloaded the front end enough to make the handling scarry but this truck is solid. If the trailer weighed significantly more than the truck this might cause a problem but when the truck is heavier than the trailer I dont see the problem. We travel forest roads and other places where traction is important and I want more not less weight on the drives. The hitch that far back is only a positive for me.

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Not really much more on a semi than the heavier RV haulers. A semi is going to push the front end whenever loaded (as a generalization). My front, loaded, is pretty heavy. An RV Hauler is still going to be running between 9400 lbs (absolute worst case measured) and 11K+ on the front (more typical). That is not going to affect anything noticeably. And that is proved out in usage scenarios. Yes, they run more weight, but not that much more. And the other forces acting on the geometry are way more extreme.

 

I disagree with this. Semis are going to run very close to 12k on the front, or if they've been built with a 12.5k or 13.2k axle, they'll probably end up in the 12k+ range. It's simple mathematics: they get paid the most the closer they are to 80k gross. They're limited to 34k on the tandem set, and limited to 34k on the trailer unless they've got >8' (38k) or >10' (40k) spread axles. 80k-34k-34k=12k, so anything less than 12k on the steer means the same amount of weight under 80k, which means the same amount of payload left on the dock. Also, the other forces acting on the geometry may be larger (~25k pin weight), the geometry is less severe (fifth wheel is likely just a bit forward of drive axle centerline, just enough to put a few pounds on the steer axle).

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As to a short singled truck, it really is not an issue. Or at least it has not proven to be, as LONG AS you have enough pin weight and do not have a lot of cantilevered force acting at the back of the trailer. Where I would worry about a short singled truck - or really any RV hauler - is when there is a very light pin weight relative to the total trailer weight, AND there is a lot of weight in the back of the trailer. Specialized Toyhaulers carrying many thousands of pounds in the garage have proven to be an issue. But that is not related to a short singling, but to the massive movement of force from the rear of the trailer acting on the front pin area. Set that in motion and you can have some "issues". In practice Henry and I have seen a few pretty severe examples of poor handling in that situation.

 

On the other hand, with a conventional trailer at 21+K lbs and 5000+ lb pin weight on a 182" wheelbase singled short Volvo 610 there were zero issues. Even running a heavier trailer at 23K and 6K+ pin I had no problems. That is over 120+K miles and 10+ years of fulltiming with those rigs. That tractor handled excellently in heavy winds and on poor condition roads. So given that other parameters were in normal ranges I see no issue with short singled trucks.

A truck that's singled short is likely going to have a longer moment arm to the RV hitch, unless the bed is too short for a Smart car or similar. That longer moment arm has the potential to create a more severe weight shift with higher pin weights. As such, I really don't like the statement "as LONG AS you have enough pin weight" - the goal with a long moment arm is to NOT load it heavily, for fear that it'll unload the front too much. Longer wheelbase then becomes an advantage. I'm glad you had zero issues in your 610, but one data set of empirical data is not enough to define a trend.

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Peety, I never said (or meant to say) that a semi has a light front end. In fact, as you said, I indicated a commercial semi tends to run close to the axle limits when loaded. Because they want to carry as much weight as possible. But the fact remains, that a 1000 lb unloading of the front axle is simply not an issue in RV use. That was the point.

 

Singled short with a long overhang in the back for the hitch certainly can be an issue if you take it to extremes. You have to balance all the factors to make for a decent handling rig. That was the original point. But within the limits of the normal HDT RV conversion there have not been issues that I know of. All of the issues I'm aware of have been when pushing the extremes. Too much overhang with heavy pins, not enough pin weight with a heavy weight in the rear of the trailer, etc.

 

To address my particular 610 configuration, that truck was singled short with a 42" overhang to the pin. A 182" wb is about as short as that truck could handle. It did not carry a car. It offloaded the front axle by about 480 lbs. I do not consider that significant. Do another configuration and you could cause issues.

 

It is the owners responsibility to understand what they are doing with their rig, and plan accordingly. That is a big part of the "purpose" of this post. We are seeing more and more trucks converted by owners for more "esoteric" purposes. They need to understand where the acceptable limits are and be aware that if you push those limits you really need to understand the dynamics. And you are right about the long moment arm - one cannot just randomly extend the pin to any length behind the axle. Use the weight and measures spreadsheet for predictive analysis and one should be fine. Offloading the fronts to the degree seen in the common RV Hauler does not seem to affect handling in any measurable way. That "common" figure is 56-60". That works fine with pin weights up to the 7500 lb range on most trucks singled mid or long. Go over that, or extend the hitch more, and you better be massaging numbers carefully. The numbers used for guidelines on weight distribution are based on the manufacturers body builders and chassis loading recommendations, and they do seem to be working in practice.

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I'm using DNAD to design a trailer with moving bogie wheels like a Landoll. Wheels back in travel position so trailer will have pin weight and behave itself on the road and not act like a cross between a weather vane and a teeter totter, and then move ahead for parking manoeuvres. Can't decide if this is easier than steerable trailer axles. See Simard steering trailer on you tube.

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A truck that's singled short is likely going to have a longer moment arm to the RV hitch, unless the bed is too short for a Smart car or similar. That longer moment arm has the potential to create a more severe weight shift with higher pin weights. As such, I really don't like the statement "as LONG AS you have enough pin weight" - the goal with a long moment arm is to NOT load it heavily, for fear that it'll unload the front too much. Longer wheelbase then becomes an advantage. I'm glad you had zero issues in your 610, but one data set of empirical data is not enough to define a trend.

 

peety3, my truck was singled short way back (in Jack's times) because that was the trend then. I didn't plan to carry any vehicles, but did plan on a massive drom so I did exactly what you are talking about pushed the hitch way back on a short singled truck.

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Here's some real life experiences:

Driving the truck after it was singled was a real tricky thing. Even under light braking on wet road it would want to swap ends, so I weighed it, it was totally out of whack, front was 11,500 lbs, the rear axle was 6,500. I needed serious weight, quickly. When I built my hitch I added a massive 1 inch thick mounting plate on it, the hitch weighed over 800 pounds and I pushed the frames back about 4 feet. Things got better solo and things got really good pulling (I still didn't have the deck on then). With the deck on the truck became safe again. Once I added the drom,

 

Kechup%20and%20A1_zps7oeitb3k.jpg

 

things got as good as they could be. If I were to do it again I would extend the frame even more by about 6 inches. I have minimum clearance between the drom and the fifth in a turn,

 

Low%20deck%20clearance_zpsj9u4kwqu.jpg

 

and can't go full 90 degrees maneuvering in tight spaces (actually I hit my owning with a drom doing it once). I have a heavy pin weight 6,500 pounds which works out to about 28% on my fifth's GVWR. I ran my rig in all kinds of combination, truck solo, with light fifth, with heavy fifth, heavy fifth plus 2,000 pounds of hitches in the drom on the truck (that got the whole thing to about 47,000 pounds going down the road), I haven't seen any adverse behavior that you are concerned about on a truck singled short and a hitch way behind the axle. Although in that last instance (at 47,000 pounds) I came close to the limit of the front axle (the hitches in the drom transferred lot of weight forward) and the limit of the singled rear axle (hitches plus the pin weight). I have been a CAT scales customer for years.

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I'm having a little trouble wrapping my brain around your comments, Phoenix.

 

 

Driving the truck after it was singled was a real tricky thing. Even under light braking on wet road it would want to swap ends, so I weighed it, it was totally out of whack, front was 11,500 lbs, the rear axle was 6,500. I needed serious weight, quickly.

 

 

Was this in a bare, bob-tail configuration? No deck, drom or hitch?

 

What is your wheelbase? I would think that shortening the wheelbase, in a bob-tail state, would ADD more weight to the rear axle.

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I'm having a little trouble wrapping my brain around your comments, Phoenix.

 

 

 

 

Was this in a bare, bob-tail configuration? No deck, drom or hitch?

 

What is your wheelbase? I would think that shortening the wheelbase, in a bob-tail state, would ADD more weight to the rear axle.

 

Yes, that was a bare bone configuration, I was ferrying the truck. Bought it in Salt Lake City, had it singled by the Volvo dealer there and drove it 3,000 miles to New Hampshire. The problem with singling short is that you remove about 2,000 pounds from the rear (an entire axle), so it goes down from about 8,500 to 6,500 and becomes a real "light ass truck" and a real short base truck. Actually the first truck conversion I did was a truck that was singled long (mine only had a hitch in frame rails at that time). I was spooked by my truck at that time, so I wanted to see what a "bare bone" singled long truck would do bobtail. The difference was night and day, I couldn't "drift" the rear end even under heavy braking.

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